A substrate formed of, for example, silicon carbide (SiC), gallium nitride (GaN) and diamond has recently been considered to be used in a power MOSFET device because this type of substrate is advantageous over silicon (Si) in terms of thermal conductivity, dielectric breakdown electric field strength, and electron saturation rate, enabling a high temperature operation of an element. As for a method of forming a gate insulating film of the power MOSFET device, it has been proposed to deposit a material capable of forming an insulating film on a substrate using a deposition method. As for the material to be deposited, a high-dielectric material is considered to be desirable and in particular, alumina (Al2O3) is attracting attention considering the physical property value such as, for example, a band gap.
Since Al2O3 has a plurality of electron traps, however, when the gate insulating film is formed of Al2O3 alone, a flat band voltage is shifted, thereby making a threshold voltage unstable. Thus, it has been proposed to eliminate the electron traps by adding nitrogen having valence electrons of which the number is intermediate between the number of valence electrons of the oxygen in the insulating film and the number of valence electrons of the silicon in the substrate. See, e.g., “Development of Defect Control Technology of High Dielectric Constant Gate That Realizes High-speed Transistor” released to the press by Fujitsu Laboratories (searched on Jul. 2, 2012) through online [Internet (URL:http://pr.fujitsu.com/jp/news/2002/07/1.html)]. In addition, when adding nitrogen by several at % to Al2O3 that forms a gate insulating film, a method of producing an AlON film by nitriding Al2O3 by plasma is used. See, e.g., “K. Manabe, IEICE TRANS. ELECTRON, Vol. E87-C, No. 1, page 30, 2004”.